Conventionally, a gas turbine has an air compressor (sometimes referred as a “compressor” hereinafter), a combustor and a turbine serve as major components, wherein the combustor is installed between the compressor and the turbine that are directly connected to each other by a main shaft; an air serving as a working fluid is inhaled into the compressor by rotation of the main shaft and compressed therein; the compressed air is introduced into the combustor and burned with a fuel; and the high temperature and high pressure combustion gas is exhausted into a turbine so as to rotary-drive the main shaft with the turbine. The gas turbine constructed in such a manner is utilized as a driving source by having a generator and the like connected to the front end of the main shaft, and is also utilized as a jet engine by installing an exhaust port for injection of combustion gas at the front of the turbine.
And now, in recent years, especially reduction of NOx in exhaust gas being discharged from a gas turbine is strongly desired for an environmental issue as one of vital legal regulations. Therefore, a combustor which actually generates NOx especially requires a technology to suppress generation of NOx. In order to achieve this, as a combustion method to be adopted to a combustor, a premixed combustion method has become a main stream, wherein a fuel and compressed air are burned after being mixed preliminarily. In this premixed combustion method, because a fuel disperses uniformly and tenuously in the compressed air, local increase in temperature of combustion flame can be prevented, thereby making it possible to reduce the generation amount of NOx which increases in accordance with an increase in temperature of combustion flame.
Here, a more common gas turbine than conventional to which a combustor using a premixed combustion method is applied will be described by referring to FIG. 47. This gas turbine 1 mainly consists of a compressor 2, a gas turbine combustor 3 and a turbine 4. The combustor 3 is installed to a casing 5 which has a cavity being formed between the compressor 2 and the turbine 4, and consists of a combustor basket 6 which has a combustion region; a transition piece 7 which is connected to the front end of the combustor basket 6; an outer shell 8 which is installed so as to be concentric to the combustor basket 6; a pilot nozzle 9 which is installed on the axis of the combustor basket 6 from the rear end; a plurality of main nozzles 10 which are installed at even intervals in a circumferential direction around the pilot nozzle 9; a bypass duct 11 which opens to the casing 5 being connected to a side wall of the transition piece 7; a bypass valve 12 which is installed to the bypass duct 11; and a bypass-valve variable mechanism 13 which adjusts the degree of opening and closure of the bypass valve 12. (See the Japanese Patent Application Laid-Open No. 2001-254634, for example.)
Being constructed as mentioned above, compressed air being compressed in the compressor 2 flows into the casing 5 (an outline arrow in the drawing), reverses for approximately 180 degrees (arrows in solid line in the drawing) after going through a tubular space which is formed by an outer circumference surface of the combustor basket 6 and an inner circumference surface of an outer shell 8, and is introduced into the combustor basket 6 from the rear-end side. Next, a fuel is blasted to the pilot burner (not illustrated) at the front end of the pilot nozzle 9 and be subject to diffusion combustion and is also subject to premixed combustion by being mixed with a fuel injected to the main burner (not illustrated) at the front end of each of the main nozzles 10, so as to become high temperature and high pressure combustion gas. The combustion gas goes through the internal of the transition piece 7 and is exhausted from the front end thereof, so as to drive the turbine 4. In addition, a part of compressed air (sometimes referred as “bypass air” hereinafter) inside the casing 5 is supplied to the internal of the transition piece 7 from the bypass duct 11, which plays a role of adjusting the density of combustion gas.
However, although the above-mentioned pre-mixed combustion method seemingly excels in reduction of NOx, it has a problem that combustion vibration is easy to occur because flame is thin and burns in a narrow region in a short time, resulting in an excessive combustion energy per unit space. This combustion vibration is generated by having a part of combustion energy converted into vibrational energy, and not only produces significant vibration and noise when it propagates as a pressure wave and resonates with an acoustical system consisting of casings of a combustor, a gas turbine and the like but also induces pressure fluctuation and heat-generation fluctuation inside the combustor, thereby making state of combustion unstable, which eventually interferes a decrease in NOx.
In order to cope with such a problem of combustion vibration as mentioned above, conventionally, by actually operating a gas turbine, appropriate adjustment is made so as to operate it in a normal condition and, at the same time, regular operational conditions are set as needed. Therefore, cumbersome adjustment activities are indispensable.
Additionally, a conventional combustor trying to reduce the combustion vibration has a resonator having a cavity installed around the outer circumference of a combustor basket and a transition piece which serve as cylinder bodies having a combustion region therein, and has sound-absorption holes opening to the cavity formed therein. (See FIG. 1 through FIG. 3 on pages 3 through 5 of the Japanese Patent Application Laid Open No. 2002-174427, for example.) By this combustor, fluid particles serving as vibration elements of the combustion vibration that occurs in the combustion region resonate with the air in the cavity inside the resonator and vibrate through the sound-absorption holes, damping the vibration amplitude thereof. In this way, it is possible to reduce the combustion vibration, thereby realizing more or less a decrease in NOx.
However, in the conventional combustor trying to reduce the combustion vibration as mentioned above, it is originally assumed that the combustion vibration occurs in a high-frequency area. Therefore, it is effective to the combustion vibration in a high-frequency area, but at the same time, it cannot be said to thoroughly cope with the combustion vibration in a low-frequency area.